1. Field of the Invention
The present invention relates to a constant current circuit, and more particularly to a current source type constant current circuit which is suited to implement in a semiconductor integrated circuit.
2. Description of the Prior Art
In a current source type constant current circuit, constant current is basically derived from a collector current of a PNP transistor operated in an active resin.
FIG. 1 shows a basic circuit configuration for driving a gate of a PNPN switch used for a speech path switch or the like, with a constant current. In FIG. 1, numeral 1 denotes a PNPN switch, A and K an anode and a cathode, respectively, of the PNPN switch 1, R.sub.1 and R.sub.2 resistors Q.sub.1 a PNP transistor, D.sub.1 a diode, E a power supply and SW a switch. In this circuit arrangement, when the switch SW is closed, if a negative voltage is applied to the cathode K of the PNPN switch, a constant current is supplied from the collector of the PNP transistor Q.sub.1. A collector current I.sub.CQ1 of the PNP transistor Q.sub.1 at this time can be approximated by: ##EQU1## where E is a power supply voltage, V.sub.BEQ1 is a base-emitter forward voltage of the transistor Q.sub.1 (which is approximately equal to 0.7 volts), and .alpha..sub.Q1 is a grounded base current gain of the transistor Q.sub.1.
However, when the circuit is to be implemented in a semiconductor integrated circuit, particularly in a high breakdown voltage semiconductor integrated circuit, the grounded base current gain .alpha..sub.Q1 shown in the equation (1) varies with manufacturing process and hence the resultant constant current significantly varies.
More particularly, in the semiconductor integrated circuit, the PNP transistor is usually a lateral transistor the current gain of which is as small as 0.3 to 0.7. Moreover, the lateral transistor is apt to be affected by surface condition of the device and hence the current gain changes significantly. Particularly when a high breakdown voltage PNP transistor is to be manufactured, an impurity concentration in a base region must be low. As a result, it is readily affected by the surface condition. Furthermore, in order to meet the requirement of high breakdown voltage, a base width must be increased. This results in small current gain (about 0.3). Because of this variation of the current gain, the magnitude of the constant current significantly changes.
As an approach to resolve the above drawbacks, a circuit configuration has been proposed in which an NPN transistor Q.sub.2 (not shown) which is easy to attain a high current gain is Darlington-connected to the PNP transistor Q.sub.1 to obtain a high equivalent current gain for minimizing the variation of the magnitude of the constant current due to the variation of the current gain.
However, this circuit configuration poses the following problems depending on the application thereof. First, when it is used in a high breakdown voltage application, the NPN transistor which is used to increase the current gain cannot provide a sufficient collector-emitter breakdown voltage because of its narrow base width (in longitudinal direction). Namely, the requirement of high current gain is incompatible with the requirement of high breakdown voltage. Secondly, when the circuit is used in a circumstance where a transient voltage is applied to the output of the constant current circuit when the transistor is non-conductive, a junction capacitance is amplified by the NPN transistor with a Miller effect so that a large transient current may flow.